I have backed up my iPhone to iTunes (via wifi) to my Mac. How do I go back and view contacts, etc. on my mac?
If you are just backing up your phone via wifi to your Mac there isn't a native tool to view the contacts. (There are some third party tools, but I don't think that is what you need). If you are syncing your iPhone to your Mac then they should show up in the contacts app.
More likely is that your contacts are being synced to whatever account you are using (gmail, yahoo, iCloud, etc...) and you just need to configure that account on your Mac and then you will see the contacts that way.
If your task is to view contacts on an iTunes backup of your iOS device when the original contacts have been lost you can either restore the backup to a different iOS device, or use apps like iExplorer to export the data you need.
[Disclaimer: I'm a satisfied, paying customer who uses iExplorer every day. I have no financial stake in Macroplant.]